Engines may be configured with direct fuel injectors that inject fuel directly into a combustion cylinder (direct injection), and/or with port fuel injectors that inject fuel into a cylinder port (port fuel injection). Direct injection allows higher fuel efficiency and higher power output to be achieved in addition to better enabling the charge cooling effect of the injected fuel.
Direct injected engines, however, can generate more particulate matter (PM) emissions (or soot) due to diffuse flame propagation wherein fuel may not adequately mix with air prior to combustion. Since direct injection, by nature, is a relatively late fuel injection, there may be insufficient time for mixing of the injected fuel with air in the cylinder. In some operating conditions, the liquid droplet may directly impinge on the combustion surfaces such as the piston, head, and liner. Similarly, the injected fuel does not encounter turbulence when flowing through the valves. Consequently, there may be pockets of rich combustion that may generate soot locally, degrading exhaust emissions. The emissions may be further exacerbated during an engine cold start operation. In particular, until the combustion chamber is fully warmed up, soot is generated due to poor fuel evaporation caused by poor fuel injector spray characteristics at low fuel rail pressure and/or fuel impacting the cold metal surfaces of the combustion chamber.
Engine testing data indicates that PM emissions can be reduced by increasing fuel rail pressure since the fuel pump is typically camshaft driven and the engine must be rotated to pump fuel. One example approach for increasing fuel rail pressure before an engine start is shown by Birch et al. in WO 2013076217. Therein, during braking of a hybrid electric vehicle operating in an electric mode, at least a portion of the negative torque is used to intermittently crank the engine, thereby improving engine lubrication and fuel rail pressure.
However the inventors herein have identified potential issues with such an approach. As an example, even with fuel rail pressure adjusted, there may not be sufficient heating of the combustion chambers to substantially reduce PM emissions during the subsequent engine restart. For example, the engine may not be rotated to a position, or maintained at a position, where sufficient heat transfer can occur. As such, engine testing data further indicates that PM emissions can be greatly reduced by engine heating. Thus even with high fuel pressure, if the combustion chamber is not sufficiently heated, there may still be soot emissions during the engine restart. In addition, due to engine start time requirements, the number of engine rotations allowed prior to a first fuel injection to the engine may be limited, limiting the increase in fuel rail pressure to below an optimum level.
In one example, some of the above issues may be at least partly addressed by a method for operating a hybrid vehicle system comprising: while propelling a hybrid vehicle via only motor torque, rotating an engine unfueled at lower than a threshold speed until a piston temperature is higher than a threshold. In this way, slow engine rotation can be used to sufficiently heat an engine before a restart, improving PM emissions from the engine during direct fuel injection on the subsequent engine restart.
As an example, while operating a hybrid vehicle in an electric mode, and while a cylinder piston temperature is below a threshold temperature, the engine may be slowly cranked, unfueled, via the hybrid vehicle's motor/generator to prepare the engine for an imminent engine start. In one example, the slow cranking may be initiated at least 2-3 minutes before an engine start. The engine is rotated slowly so that all the cylinders get warm as they go through respective compression and expansion strokes. As such, each compression stroke of the engine causes the compressed air to get hot and transfer heat to the cylinder head and piston. Even though the absolute amount of heat transferred to the engine may be low, the heat is transferred directly to a location where heating reduces soot emissions. Thus, during the slow rotating of the engine unfueled, each engine cylinder is heated via compression stroke heating. As such, the engine may be rotated at a lower than threshold speed for the pre-heating via the motor/generator of the hybrid vehicle. In particular, the engine may be rotated slower than the engine would be rotated via a starter motor during engine cranking prior to a restart. For example, during a typical starter motor start, the engine may be cranked at 150 rpm, while during the slow cranking via the hybrid vehicle motor, the engine may be initially cranked at 10 rpm (to the first position) and then cranked at 30 rpm to subsequent positions. Additionally, during the slow cranking, an intake throttle may be maintained closed so that the compressed aircharge is pulled back into the engine with no net flow to the exhaust. Optionally, an EGR valve may also be opened so as to recirculate flow back to the engine and reduce engine vacuum. By slowly rotating each cylinder through a compression stroke, the engine acts as a heat pump and at the bottom of the exhaust stroke, the cylinder aircharge may become cooler than ambient. However, over each cylinder cycle, a net cylinder piston heating may result. Once the engine cylinders have been sufficiently warmed, and the piston temperature is above the threshold, cylinder fuel injection may be resumed to restart the engine. In some examples, after the initial engine slow rotation, the engine may be further rotated to preposition the engine for the imminent engine start. For example, the engine may be rotated to a position that improves engine restartability before cylinder fueling is resumed.
In this way, an engine may be rotated slowly so that the heat from compression is given time to heat the cylinder chamber. By slowly turning an engine for an extended period prior to an engine start, heat generated during a cylinder compression stroke can be transferred to cylinder walls and used to heat the engine in anticipation of an engine start. By pre-heating the engine, particulate emissions from the engine can be reduced, particularly during an engine cold-start. In addition, fuel pressure can be raised to an optimum value for the start, improving fuel injector spray characteristics during the restart. Overall, cold-start emissions can be improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.